The present invention relates to an electronic switching circuit for reducing power-on switching transients.
Number 1 in FIG. 1 indicates a known electronic switching circuit in which a first solid-state electronic switch 3, e.g. defined by an IGBT transistor, has a first terminal 3a connected to a voltage source V.sub.al via an inductor L.sub.1 a second terminal 3b connected via an inductor L.sub.2 to a first terminal of a load 5 shown schematically by an inductor Lc and a resistor Z.sub.c connected in series with each other; and a control terminal 3c conveniently defined by the gate terminal of the IGBT transistor, and which is supplied via a control resistor R.sub.g with a control signal C. The binary control signal C (FIG. 2a) may be defined by a voltage varying between a first logic state (e.g. a zero or negative voltage) corresponding to opening of electronic switch 3, and a second logic state (e.g. a positive voltage V.sub.c) corresponding to closing of electronic switch 3; and electronic switch 3 is connected in parallel to a recirculating diode D.sub.c1 having the cathode connected to terminal 3a and the anode connected to terminal 3b.
The electronic switching circuit also comprises a second solid-state electronic switch 7 also defined by an IGBT transistor, and which has a first terminal 7a connected to the first terminal of load 5; a second terminal 7b connected to a reference voltage Vref to which a second terminal of load 5 is also connected; and a control terminal 7c supplied via a control resistor Rg with a control signal preferably but not exclusively opposite to control signal C. Electronic switch 7 is also connected in parallel to a recirculating diode D.sub.c2 having the cathode connected to terminal 7a and the anode connected to terminal 7b.
The above circuit may conveniently define a CHOPPER for dividing the direct supply voltage Val and supplying load 5 (e.g. comprising an electric motor) with a pulsating voltage; and the CHOPPER circuit may be combined with another of the same type to supply a load with alternating current and so define an INVERTER.
Known electronic switching circuits present a drawback when turned on, due to the physical behaviour of diode D.sub.c2. That is, when switch 3 is open and switch 7 closed (low logic value of control signal C), the loop defined by load 5 and recirculating diode D.sub.c2 is supplied with recirculating current by inductor L.sub.c forming part of the loop; and, when switch 3 is closed, diode D.sub.c2 is reverse biased and should therefore be turned off. In actual fact, however, when switch 3 is turned on, the current Ic supplied by diode D.sub.c2 decreases substantially steadily to begin with (first portion T1 in FIG. 2b), and, on reaching the zero value I.sub.co (at which it should stop decreasing if diode D.sub.c2 were to perform ideally), continues to fall (portion T2) to a negative value I.sub.r from which it then returns to the zero value I.sub.co, thus creating in diode D.sub.c2 a negative current peak Ir induced by reverse conduction (recovery) of the diode. As a result, the current I.sub.g supplied by switch 3 and equal to the sum of the load current and current I.sub.c of the diode (FIG. 2c) increases substantially steadily at portion T1, and, on reaching the steady-state value (at which it should stop increasing if diode D.sub.c2 were to perform ideally), continues rising (portion T2) up to a positive value I.sub.p from which it then falls back to the steady-state value, thus creating in switch 3 a positive current peak I.sub.p induced by reverse conduction of diode D.sub.c2.
Diode recovery is a well known phenomenon which has always been considered uncontrollable, and which, on account of the current peak Ir applied to diode D.sub.c2 and the normally high supply voltages of such electronic circuits, results in the generation of extremely high instantaneous power capable of destroying the diode. For example, supply voltages of thousands of volts (e.g. 2000 V) may result in recovery currents of about a thousand amperes (e.g. 1500 A) and an instantaneous power of several megawatts (e.g. 3 MW) which no diode on the market could withstand. Similarly, the high current supplied by switch 3 may either damage the switch itself or at least cause it to operate, albeit for a few instants, outside the safety range.
The known solution to the above drawbacks is to prolong the turn-on time of electronic switch 3 to gradually reduce the current in diode D.sub.c2 and so achieve lower recovery current values by selecting a sufficiently high resistance of resistor R.sub.g. Electronic switch manufacturers, in fact, specify a minimum resistance of resistor R.sub.g for safeguarding against the recovery phenomenon. Prolonging the turn-on time of electronic switches, however, clearly results in a drastic increase in the amount of energy dissipated each time the circuit switches.